1. Field of the Invention
This invention relates to the thermal cracking of a hydrocarbonaceous material in a pyrolysis furnace. More particularly, this invention relates to reducing coke fouling during the transfer of the cracked product from the furnace through a shell and tube type, first heat exchanger encountered by that product after leaving the furnace.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene toluene, and xylenes. In an olefin production plant, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to severe hydrocarbon thermal cracking at elevated temperatures (1,450 to 1,550° F.) in a pyrolysis furnace (steam cracker or cracker).
The cracked product effluent (product) from the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule or C1 to C35). This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate and individual product streams of high purity, e.g., hydrogen, ethylene, and propylene. After the separation of these individual streams, the remaining cracked product contains essentially hydrocarbons with four carbon atoms per molecule (C4's) and heavier (C4+). This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5+ stream is removed as a bottoms product.
The hot, cracked furnace product, upon leaving the furnace, is first introduced into a tube-type heat exchanger wherein, for example, boiler feed water is indirectly heat exchanged with the hot product stream to cool the product to a more manageable level, and to generate high pressure steam for use elsewhere in the plant. The tube type heat exchanger (exchanger) employed is a unit that contains a plurality of heat exchange tubes, e.g., typically from about 50 to about 100 tubes. The number of tubes varies widely depending on a number of variables such as exchanger and tube internal diameters. The tubes ends are spaced apart by a metal member that is termed a tube sheet face.
The transfer (conduction) of product from the furnace to the exchanger is accomplished through a transfer line and a truncated cone adapter which expands from the smaller diameter transfer line to the larger diameter exchanger. The truncated adapter is typically refractory lined incorporating various conical or trumpet style designs intended to distribute the flow evenly across the larger diameter. In the Figures hereinafter the refractory is not shown for sake of clarity since it is well known that the adapter will be refractory lined. The mass flow rate (pounds/second/square foot) of product from the furnace through the transfer line and cone, and into and through the exchanger tubes is relatively constant under normal conditions. The exchanger is an elongated unit, since the tubes in its interior are long in order to achieve as much heat transfer from the product to the boiler feed water as reasonably possible, thereby producing optimal amounts of boiler feed steam.
Steam is expensive, so it is desirable to make as much steam as possible from the hot product before other processing of that product. Accordingly, it is desirable to extract as much steam making capability as possible from the first exchanger that is encountered by the product.
One way to increase the steam generating capability of an existing tube-type exchanger is to increase the length of the tubes, hence, the length of the exchanger itself. Often, due to physical restrictions within the plant, it is not practical to extend an exchanger longitudinally to provide for longer tubes therein.
Another way to increase the steam generating capability of an existing tube-type exchanger is to increase the number of individual tubes thereby enlarging the interior volume of that exchanger. Although this can increase the amount of steam recovered from that exchanger, since the furnace and transfer line are unaltered, the adapter cone angle will be increased for the larger tube sheet diameter and the mass flow rate of product through each individual exchanger tube will be reduced. This can lead to plugging of some or all of the tubes with a hard, graphite-like coke deposit (coke) which in turn reduces the steam generating capability of the exchanger and, ultimately, requires shut down and clean out of the exchanger tubes, a time consuming (typically 7 to 10 days) and expensive effort. When the diameter of the large end of the inlet cone becomes too large the re-circulation and low mass flow in the outer perimeter regions of the cone result in coke formation and deposition in those regions. The industry standard for mass flow through the tubes of an exchanger is from 6 to 10 pounds per second per square foot of tube cross section. This is what is thought to be needed in order to produce an optimal steam output level from an exchanger.
Accordingly, it is desirable to be able to design and operate a heat exchanger with increased steam generating capacity without risk of increased exchanger internal coke plugging even when operating with reduced mass flow of furnace product through the exchanger, and that is what this invention accomplishes.